Backward wave oscillator having anode-sole spacing of 0.05 wavelength



June 29, 1965 J. F. HULL 3,192,434 BACKWARD WAVE OSCILLATOR HAVING ANODE-SOLE SPAGING OF 0.05 WAVELENGTH Filed Feb. 9. 1960 United States Patent 3,192,434 BACKWA WAVE OSCILLATGR HAVING ANODE-SGLE SPACING 0F 0.05 WAVELENGTH Joseph F. Hull, Redwood City, Calif., assignor to Litton Precision Products, Inc, a corporation of Delaware Filed Feb. 9, 1960, Ser. No. 7,598 3 Claims. (Cl. 315-393) This invention relates generally to backward traveling wave oscillator devices and more particularly to backward traveling wave oscillators having crossed electrostatic and magnetic fields in which the frequency of the oscillation of the device is varied by means of a variable potential imposed upon the sole electrode thereof.

In the prior art that particular form of backward wave oscillator having crossed electrostatic and magnetic fields, commonly known as an M-type backward wave oscillator, has been constructed to include six basic elements, namely, a slow-wave structure, an associated sole electrode, a cathode, a grid electrode, an accelerating electrode, and means for supplying a static magnetic field. Generally, the construction of these oscillators is such that the sole and the delay line are both curved and parallel to each other or more specifically are both circular and concentric with one another, forming an electron and electromagnetic wave interaction space therebetween. Moreover, there is also applied between the sole and delay line a potential difference so that there is provided crossed electrostatic and magnetic fields within the interaction space of the device, causing an electron beam generated by the cathode to flow through the interaction space in coupled relationship with the slow wave structure and the backward traveling wave of high frequency energy propagated thereby.

In the prior art the frequencies of such oscillators have traditionally been changed by cathode modulation, or stated in another manner, by varying the potential difference between the cathode and the slow wave structure, while in general the potential diiferences between the accelerator, the grid, and the sole with respect to the cathode potential have been maintained at fixed values. As a consequence, it canlbe demonstrated that this mode of operation requires a preselected and relatively close spacing between the sole and the slow wave structure, this relatively close spacing being required for any given set of voltage and current conditions, to maintain the broadband electron optics of the oscillator so as to provide efiicient coupling between the beam and the wave energy which is propagated by the delay line over the entire electronic tuning range of the tube.

The term broadband electron optics as herein employed is intended to define the condition which exists in an oscillator when the elements of the electron gun and the elements of the interaction space are in a given space relationship to one another and have potentials imposed thereon which are selected to minimize disturbances of the electron beam from its optimum adjusted path within the interaction space as the potentials of the elements are varied in electronically tuning the tube.

Continuing, it has been demonstrated that the cathode of the prior art devices will draw relatively high currents,

typically 275 milliamperes over a potential range from 2500 to 5000 volts, for example, requiring a power supply of large physical size and weight. Such power supplies are generally very costly and difiicult to regulate, especially because of the fact that the variable voltage for tuning the tube is also the source of the electron beam. Thus, for high power operation, tuning by the conventional method of varying the cathode-to-slow wave strucpotential-becornes very undesirable from the standpoint of equipment cost, Weight, and voltage regulation. This watts or less.

is especially true when these oscillators are employed in airborne equipments.

It has been recognized in the prior art that the problems encountered in cathode modulation could be overcome if it were possible to satisfactorily modulate the s-ole-to-slow wave structure voltage. However, no knownprior art device, which has a relatively close sole-to-slow wave structure spacing, is capable of satisfactory sole modulation over a wide frequency range at high power levels, since it has been impractical and in some instances impossible to build a device capable of sole tuning because no rigorously mathematical formulas are available to permit .the simultaneous prediction of the exact eliect-s of the various parameters of the device as the elements are moved in space relationship to one another and as the voltages applied to these elements are varied. Stated in another manner, the space relationship of the elements of the device and the voltages applied thereto are interdependent so that a change in one may adversely affect the performance of the others and vice versa. Inasmuch as the various parameters of the device are mathematically interrelated and since no mathematical solution is available to completely predict their behavior, an understanding of the elfects produced by one element on the others can be obta-ined only by experimentation and logical deduction.

It has been found through experimentation on prior art devices that whenever attempts have been made to swing the sole-to cathode voltage through a wide potential range, such as 1500 to 3000 volts for example, it has disturbed the broadband electron optics of the device, causing the sole to draw abnormally high currents, as for example percent greater than normally experienced during cathode tuning of the device. In addition there is experienced a substantial drop in output power, on the order of 50 percent.

In order to more fully appreciate the mode of operation when prior art devices are sole tuned, it is worthwhile to note that many tests have been conducted on conventional X-band M-type backward wave oscillators which have the relative close sole-to-slow wave structure spacing of 0.047 inch, and such devices can be tuned over a 400 megacycle range by varying the sole-to-cathode voltage within the range of 1300 to 2700 volts. It is also worthwhile to note at this point that such a spacing is about 20 percent less than the spacing employed in an X-band device of the present invention. Under these conditions the operation of the devices was critical and the sole current was at least 5 milliamperes while the average power output was 100 The high value of sole current and the low value of power which occurs with such prior art devices has made their use undesirable and impractical for use as sole tuned tubes.

The present invention obviates the foregoing and other obvious disadvantages of the prior art crossed field backward wave oscillators which are tuned by varying a cathode-to-slow-wave-structure potential, and provides an oscillator which utilizes a preselected sole-to-slow-wave structure separation substantially larger than that heretofore employed to permit sole potential tuning of the device. More particularly, sole tuning is accomplished by varying or modulating the potential imposed upon the sole electrode by suitable potential means, while simultaneously holding the potentials of the accelerator, the grid, and cathode at predetermined fixed values. According to another embodiment of the invention, it may be desirable to operate the oscillator by varying the potential of the accelerator simultaneously with the sole potential.

In accordance with the basic concept of the present invention, there is provided a backward wave oscillator capable of providing substantial energy with good efficiency and a method for tuning said oscillator by varying the potential between the sole electrode and the cathode 3. without drawing any appreciable sole current. it has been demonstrated that an oscillator constructed with its soleto-delay line structure separation at predetermined distance larger than that heretofore considered practical in a prior art oscillator of this type, for example 29 percent larger, may be tuned over a frequency range substantially as wide as a cathode modulated device ofthe prior art, typically 1250 megacycles at X-band. Moreover, the

separation between the sole and the delay line is such that variations in the sole-to-cathode potential over a wide potential range will not change the electron optics of the device significantly nor-will it reduce its efficiency of operation.

Although such an oscillator is also capable of providing several hundred watts of average power output at high eificiency, which is as high or higher than that of prior art cathode modulated devices, it requires that only several milliamperes. of sole current be varied over a Wide voltage range to tune the device, whereas the prior art devices for the same amount of power, require several hundred milliamperes or" cathode current be varied over a similar voltage range to tune the device. Thus, it be appreciated that the sole potential of the backward wave oscillator of the present invention swings at a current level which is several orders of magnitude less that that required by the cathode of the prior art backward wave oscillators. The advantages derived from the.-

mode of operation disclosed by the present invention become more fully appreciated when it is realized that this mode of operation is accomplished at such a very low sole current level, thereby requiring a variable power supply which is much easier to regulate in comparison with the electrical equipment employed with prior art devices. Furthermore, the cost of such equipment utilized in accordance with the present invention is much lower than that heretofore employed by the prior art. 7 p

' It is therefore an object of the invention to provide a high power crossed field backward wave oscillator which is continuously tunable over a wide frequency range by vary ng the sole-to-cat hcde potential.

Another object of the invention is to provide a backward wave oscillator capable of being turned by varying the sole-to-cathode potential wherein the power requirements of an associated power supply are minimized.

A further object of the invention is to provide a sole electrode-to-delay line spacing in which sole tuning is accomplished at minimum sole current levels.

Still another object of the invention is to provide a method and apparatus in which the accelerator potential is varied simultaneously with the sole potential to correct the broadband electron optics of the backward oscillator as the electric fields of the-interaction space vary.

Still a further object is to provide a method and ap 'paratus for tuning a backward wave oscillator by means of modulating the sole electrode potential in which the associated electrical equipment is minimized in size,-

weight, and cost of voltage regulation.

The novel features which are believed to be characteristic of the invention, both as to its organization and ethod and of operation, together with the further objects and advantages thereof, will be better understood fromthe following description considered in connection with the accompanying drawing in which one embodiment of the invention is illustrated by way of example. It is to be expressly understood however, that the drawing is intende for the purpose of illustration and description only, and is not intended as a definition of the limits of the invention.

FIGURE 1 is a schematic diagram illustrating the conventional technique employed with M-type backward wave oscillators of the prior art for tuning by means of cathode potential modulation;

FIGURE 2 is a schematic diagram illustrating the man nor in which M--type backward wave oscillators may be tuned by means of sole electrode potential modulation, in accordance with the invention; and 7 FIGURE 3 is a schematic diagram illustrating one technque which may be employed with the circuit of FXG'URE 2 to provide simultaneous variation of sole and accelerator potentials by means of a common potential applied across a tapped voltage dividing network.

Referring now to the drawings, wherein the same referencecharacters designate like or corresponding parts throughout the several views, there is shown in FIGURE 1 a schematic diagram of an M-ty'pe backward wave oscillator, generally designated 8, which has associated therewith a source of DC. magnetic field descriptively illustrated by the encircled crosses, and a number of associated potential sources which are applied to the various elements of the oscillators. As shown in FIGURE .1, the oscillator includes a cathode it), a beam forming electrode i2 hereinafter referred to as a grid electrode partially circurnjacent tothe cathode, an accelerator elecposed trode 14 adjacent the cathode and the grid and forming therewith an electron gun for the oscillator, a sole electrode 16 adjacent the cathode and the grid, and a slow wave structure 17 positioned adjacent'the accelerator at the output end of the device and parallel to the sole .loand forming an electron and electromagnetic wave interaction space it? therebetween. in addition, there is also provided an output means 22, which may have any design suitable for transferring or extracting microwave enrgy generated within the oscillating system to an Xternal load circuit. The magnetic source, which is not shown but is represented by the encircled crosses,

provide magnetic fiux lines within the interaction space of the device and normal to the plane of the, drawing. The oscillator is completed by an attenuator 24 which is disposed at one end of the delay line for absorbing high frequency reflected signals on theslow wave line and a collector 25 disposed at the remote end of the interaction space for collecting residuary electrons'whic'h do not impinge upon the slow wave structure or the sole electrode. For purposes of simplicity the evacuated tube envelope is shown by the dotted line 28.

It should be noted at this point that the ocillators shown in the accompanying drawings are in the form of .a linear device for clarity of understanding. However, in prac tice these devices are constructed preferably in a circular configuration to provide a device of smaller physical size and'of more rugged construction.

Turning now to the details of the elements shown in FIGURE 1, it will be noted that the voltages imposed upon grid electrode 12, accelerator electrode 14, and sole electrode lddesignated E E and 3,, respectively, have llXfiCl values with resp ct to the cathode potentialE which is shown as a variable source. It should be noted further that the se aration between sole electrode, to, and delay line 1'7, which isdesignated d has conventionally been relatively small, on the order of 0.04 of a wave length at X-band, for example. In the conventional M-type backward wave oscillator of the prior art, the spacing d is required to be a predetermined small distance which is consistent, with the broadband electron optics of the deviceas determined by position of cathode in, grid 12, and accelerator with respect to one another, the potentials imreon, and the ratio of radial-toaxial DC. fields which exist between the sole anddelay line.

Consider now the diagrammatic showing of FIGURE 2, on the other hand where the elements illustrated have the same designations as corresponding elements shown in FEGURE 1. Referring to FlGURE 2, it can be seen that four basic changes appear in the drawing which distinguish this structure from that shown in FIGURE 1, namely,-the spacing between the sole electrode i6 and the slow wave structure 17, here designated 4 has been increased by approximately twenty percent, the accelerator potential source E and the sole potential source B are now vari able in lieu of being fined potentials as shown'in FlGURE 1, and the cathode potential source E is now fixed rather than being a variable source as disclosed in FIGURE 1.

As shown in FIGURE 2, the negative side of the fixed potential source E is connected to the cathode and the positive side is grounded so that the cathode is a predetermined negative value below ground. Since slow Wave structure 17 is grounded, it is therefore at a positive potential with respect to the cathode. The negative side of the second fixed potential source E in turn, is connected to the grid 12 and the positive side to the cathode so that the grid is negative with respect to the cathode 10, while the negative side of the third fixed potential source E is connected to the cathode l0 and the positive side to the accelerator. 14 so that the accelerator is also positive with respect to the cathode. The negative side of variable potential E on the other hand, is connected to the sole electrode 16 and the positive side is connected to the cathode 163 so that the sole 16 may be made variably negative with respect to the cathode 10 and also with respect to the slow wave structure 17.

In the region of the electron gun the electric and magnetic fields combine to cause the electrons to be drawn from the cathode in a cyclodial path and to be injected into the interaction space between the sole and the slow wave structure. The potential source E connected at one end to the sole electrode 16 then establishes the magnitude of the electric field between the sole and the slow wave structure, this field combining with the magnetic field to complete the electron optics for the device.

The collector 26 which is disposed at the end of the interaction space remote from the electron gun end functions to collect residuary electrons which do not impinge upon the slow wave structure or the sole electrode, while a suitable output means 22 is again coupled to the slow wave structure near the gun end of the slow wave structure for'transferring backward wave energy to an external load circuit. The tube is completed by an attenuator 24 disposed at one end of the slow wave structure for absorbing high frequency wave energy propagated along said line in the same direction as the electron beam, thereby substantially preventing reflection of forward traveling waves from the other end and thus rendering said structure electrically aperiodic within the limits of the frequency band.

Before discussing the electrical operation of the oscillator and circuitry shown in FIGURE 2 and the advantages provided thereby, consideration will be given first to the mechanical and electrical problems which are overcome through the use of an increase in spacing between sole electrode 16 and slow wave structure 17 and in tuning such oscillators by varying a sole potential source. Firstly, from a mechanical point of view, it will be recognized that an increase in the sole-to-slow-wave structure spacing will permit the electron beam to be disturbed much more than was possible with prior art oscillators without increasing the sole current drawn. Stated in another manner, the sole is initially further away from the slow wave structure than it is in the conventional devices so that the electron beam may be disturbed without causing the electrons to impinge upon the sole, and thereby reducing the sole current that would otherwise be drawn. It can be demonstrated that by proper selection of a sole-to-slow-wave structure spacing and the provision of broadband electron optics, sole current on the order of 3 milliamperes or less will be drawn.

Secondly, from an electrical point of view, the use of the sole tuning method of the invention with its increased sole-to-slow-wave structure spacing eliminates the necessity for modulating the cathode high power voltage supply at high current levels. Finally, it will also be apparent to those skilled in the art that the use of the sole-to-sloW-wave structure and sole tuning techniques as taught by the present invention reduces dramatically the size and weight, and the cost of the electrical equipment employed to modulate and regulate such backward wave oscillators.

It should be noted at this point that it is well known in the present art that the frequency of a backwardwave oscillator of the present type is a function of the ratio of the electric field to the magnetic field (E/B). In general, the magnetic field (B) is substantially constant, and is provided by a suitable means such as a permanent magnet, for example, whereas the electric field (E) is variable depending upon the value of the potential difference applied between the sole and slow wave structure and the distance separating these two elements.

With a given potential difference between the sole and the slow wave structure and a given separation therebetween, a predetermined electric field is established. Since the distance (1 between the slow wave structure and the sole normally remains fixed, the electric field (E) will remain constant if the potential applied thereacross is held constant. However, by varying the potential between the sole and slow wave structure, the electric field is changed which in turn changes the frequency of the device.

Consider now the electric operation of the backward wave oscillator in which there is provided an increase in the sole-to-slow-wave structure spacing and in which the sole potential modulating technique is employed as taught in the present invention. The operation of the tube according to the invention is based upon the inte-raction which will occur between the beam and the wave energy propagating in the slow wave structure in the backward direction. Stated in another manner, the wave energy travels in the opposite direction to the beam when the apparent or phase velocity of one of the components of this wave is substantially equal to and in the same direction as the velocity of the beam.

In operation cat-ho de 10 is activated to provide a source of electrons which are drawn therefrom by a positive potential applied to accelerator 14. As the electrons leave cathode it grid 12 functions asa means which helps to form an electronbeam which, under the influence of both electric and magnetic fields, follows a cycloidal path. At the peak of the cycloid, the electrons are then injected into the interaction space between the sole electrode and the slow wave structure. For a given set of electric fields in the gun region and in the interaction space these electrons are injected into the interaction space in tight coupling relationship with the Wave energy propagated by the slow wave structure.

Assuming that the potential dilferences between sole and delay line remains constant, the electric field (E) therebetween' would also remain constant. However, it will be recalled that the electric field (E) may be changed by varying the potential difieren-ce between the sole and the slow Wave structure. Thus, under such a mode of operation the potential (E applied to the sole is varied and the electric field ('E) changes correspondingly. This change in (E) changes the ratio of the electric field to the magnetic field (E/B) so that the frequency which is a function of this ratio also changes. Further changes in the potential difference between sole and delay line result in further changes in the frequency. Consequently, the device is continuously tunable by varying the sole-to-cathode potential provided the range of potential variation is compatible with the sole-to-slow-wave structure space d As the sole-to-cathode voltage is varied, the amount of current drawn by the sole is small, since the distance between the sole and slow wave structure is sufficiently large enough for a given potential range to permit the voltage to swing through the range and avoid distorting the electron optics of the device. Under these conditions, the number of electrons which impinge upon the sole and which in turn cause sole cur-rent to be drawn is minimized. In operation, this means that the sole voltage may be varied over a substantially wide range at small current levels and requires a minimum amount of power for tuning purposes. This characteristic of the sole power supply in turn means that the size, weight, and cost of voltage regulation of the power supply is at a minimum.

Itis expressly understood, of course, that the invention may be practiced in manners other than that shown in the embodiment of FIGURE 2 'by employing a circuit of the general form shown in FIGURE 3 or by other suitable means for modulating simultaneously the potential of the sole and accelerator of the device. With reference to FIGURE 3, for example, there is shown a voltage divider circuit which may be employed to provide a means for simultaneously varying 'both the sole voltage and the accelerator voltage in accordance with variations in a variable voltage signal E to thereby com pensate the electron optics of the gun region as the electron optics of the interaction space are varied. More particularly, the leads designated by arrows to elements 10, 112, 1a and 16, are connected to their respective elements replacing the potential sources designated E and E shown in FIGURE '2. The circuit illustrated in FIG- URE 3, enables one to simultaneously vary the potential applied to sole to and accelerator Ji l when the linear or nonlinear variable potential source '3 is connected to terminals 3% and 32. It may be noted, that the polarities of terminals 3% and 32 are designated positive and. negative respectively, which corresponds to the same relative polarities for the sole l4 and accelerator 16 as that shown in FIGURE 2, and serves to simultaneously vary the electric field between the sole and delay time, and between the accelerator and cathode in the same sense.

It has been found that the utilization of the basic concepts herein set forth may be employed to provide M-type backward wave oscillators whose sole-to-slowwave structure separation is greater than that heretofore employed, for example 0.05 of a wavelength with respace to the Wavelength of the center frequency of the band at X-band, w i ch permits the sole-to-cathode .potential difference to be varied or modulated to effectuate voltage tuning of the device. in addition, there is provided a means for simultaneously varying the electron optics in the gun region of the device as the soleto-slow-wave structure electric fields are changed, substantially eliminating distortion of the electron optics Within the interac-tion space. This minimizes the current drawn by the sole circuit and helps maintain the efficiency of the device.

While the oscillator has been described with reference to only one particular sole-to-slow-wave structure separation and with reference to only two particular types of potential sources applicable to the various elements, it will be understood that various modifications could be made in the apparatus and circuitry thereof Without departing from the spirit and scope of the invention. Accordingly, it is. to be expressly understood that the foregoing description shall be interpreted only as illustrative of the invention, and that the appended claims be accorded as broad an interpretation as is consistent with the basic concept herein taught.

What isclaimed as new is:

l. A voltage tunable backward traveling wave oscillator tube of the type having crossed electric and magnetic fields, said oscillator comprising: an electron gun including an electron emissivecathode a grid electrode substantially circumjacent to said cathode for initially shaping an electron beam from electrons emitted from said cathode, and means including an accelerator electrode adjacent said cathode and grid for providing electron optics for forming and directing an electron beam in a predetermined direction; a delay line having a geometrically periodic structure for supporting backward traveling wave energy; a sole electrode substantially parall-e1 to. said delay line and spaced therefrom by a distance of about 0.05 of a wavelength or greater with respect to the center frequency of the operating frequency band of said oscillator and forming an interaction space therewith for receiving the electron beam; means for collecting electrons from said beam; means for providing a magnetic field in the region of said electron gun and interaction space; means for biasing said cathode at a relatively fixed negative direct current potential'with respect to ground and for biasing said grid at a relatively fixed negative direct current potential with respect to thecathocle potential; means including a variable potential source which is negative with respect to the oath ode potential for varying the frequency of the operating frequency band of said oscillator by controlling the electric field gradient within said interaction space; and means for simultaneously imposing a variable positive potential upon said accelerator with respect to the cathode potential, said positive potential being variable as a continuous linear function of said electric field gradient to substantially preserve the electron optics of the oscillator tube as the sole potential is varied, and to provide relatively high output power substantially across the entire operating frequency band of the tube.

2. A backward wave oscillator tube for generating wave energy which is continuously adjustable over a relatively wide frequency range, said oscillator comprising: an electron gun including .a cathode, means including an associated accelerator electrode for controlling the number of electrons emitted from said cathode, said gun being operable to form and direct an electron beam of substantially constant intensity in a predetermined direction; a delay line having a geometrically periodic structure in the form of a symmetricalinterdigital structure for supporting backward traveling wave energy; a sole electrode substantially parallel to said delay line and being spaced therefrom by a distance of at least 5 per-cent of a wavelength with respect to the center frequency of the operating frequency band of said tube, said sole and delay line forming an electron and electromagnetic wave interaction space th-erebetween, said interaction space being oriented wi-th respect to said electron gun for neceiving said electron beam; a collector electrode at the end of said interaction space remote from said electron gun; means for producing mutually perpendicular unidirectional electric and magnetic fields within said interaction space; means for maintaining said cathode at a relatively fixed negative direct current potential with respect to ground; meanstinc-luding a variable potential negative with respect to a cathode potential for varying the intensity of the electric field within said interaction space to provide a variation of the operrating frequency over a frequency rangein excess of 15 percent of the center frequency of the operating frequency band of said oscillator; means 'for extracting wave energy from said delay line and means for imposing a variable positive potential upon said accelerator with respect to the cathode potentiahsaid positive potential being variable as a continuous linear function of said electric field gradient.

3. A backward wave oscillator tube :for producing wave energy which is continuously adjustable over a relatively wide frequency range, said oscillator comprising: an electron gun including a cathode electrode, means including a grid electrode disposed in spaced relationship with said cathode for initially shaping an electron beam from. electrons emitted from saidcathode; and means including an accelerator electrode and said cathode and grid electrodes to provide a broadband electron optical system for forming and directing an electron beam in a predetermined direction; a delay line in the form of a symmetrical interdigital structure andhaving a circular configuration along the length of the structure for propagating electromagnetic wave energy; a sole electrode disposed concentric to said delay line, said delay line and sole forming an interaction space for receiving the electron beam and being spaced apart by a distance greater than 5 percent of a wavelengthwith respect to a as the middle frequency of the operating frequency band of the tube to maintain said broadband electron optics; means for producing a magnetic field in the region of said electron gun and said interaction space; means for applying to said cathode a relatively fixed negative potential with respect to said delay line; means for varying the gradient of the electric field between said interdigital structure and said sole electrode to vary the frequency of electromagnetic Wave induced on said delay line over a range greater than 15 percent of the middle frequency of the operating frequency band Without a substantial drop in output power across said frequency band; means for simultaneously imposing a variable positive potential upon said accelerator With respect to the cathode potential, said positive potential being variable as a continuous linear function of said electric field gradient; and means adjacent said delay line and accelerator electrode for transferring Wave energy generated Within said tube to an external load circuit.

References Cited by the Examiner UNITED STATES PATENTS Lerbs 315-3.5 X Iskenderian 3l53.5 X Huber 3153.6 Reverdin 3l53.5 Denoh 315-393 X Dench et a1 '332-l7 Epsztein 3153.5 Denc'n 331-82 Dench 315-35 Klein 3153.6 Dench 31539.3 Rapuano 332-25 X GEORGE N. WESTBY, Primary Examiner.

ARTHUR GAUSS, BENNETT G, MILLER,

Examiners. 

2. A BACKWARD WAVE OSCILLATOR TUBE FOR GENERATING WAVE ENERGY WHICH IS CONTINUOUSLY ADJUSTABLE OVER A RELATIVELY WIDE FREQUENCY RANGE, SAID OSCILLATOR COMPRISING: AN ELECTRON GUN INCLUDING A CATHODE, MEANS INCLUDING AN ASSOCIATED ACCELERATOR ELECTRODE FOR CONTROLLING THE NUMBER OF ELECTRONS EMITTED FROM SAID CATHODE, SAID GUN BEING OPERABLE TO FORM AND DIRECT AN ELECTRON BEAM OF SUBSTANTIALLY CONSTANT INTENSITY IN A PREDETERMINED DIRECTION; A DELAY LINE HAVING A GEOMETRICALLY PERIODIC STRUCTURE IN THE FORM OF A SYMMETRICAL INTERDIGITAL STRUCTURE FOR SUPPORTING BACKWARD TRAVELING WAVE ENERGY; A SOLE ELECTRODE SUBSTANTIALLY PARALLEL TO SAID DELAY LINE AND BEING SPACED THEREFROM BY A DISTANCE OF AT LEAST 5 PERCENT OF A WAVELENGTH WITH RESPECT BAND TO THE CENTER FREQUENCY OF THE OPERATING FREQUENCY BAND OF SAID TUBE, SAID SOLE AND DELAY LINE FORMING AN ELECTRON AND ELECTROMAGNETIC WAVE INTERACTION SPACE THEREBETWEEN, SAID INTERACTION SPACE BEING ORIENTED WITH RESPECT TO SAID ELECTRON GUN FOR RECEIVING SAID ELECTRON BEAM; A COLLECTOR ELECTRODE AT THE END OF SAID INTERACTION SPACE REMOTE FROM SAID ELECTRON GUN; MEANS FOR PRODUCING MUTUALLY PERPENDICULAR UNIDIRECTIONAL ELECTRIC AND MAGNETIC FIELDS 